Resistance-sensing system with fail-safe circuit

ABSTRACT

A control circuit suitable for use in a forced-draft gas-fired heating system permits a solenoid gas valve to be turned on only if a blower is operating. The solenoid of the gas valve is controlled by a silicon-controlled rectifier (SCR) which receives its gate signal from a thermistor circuit. The blower, when operating, blows air over the thermistor to limit the extent to which it heats up in response to current passed through it, thereby maintaining the thermistor resistance high enough to operate the SCR and the gas valve so long as the blower operates properly. If the blower does not operate properly, the thermistor warms up further, its resistance decreases markedly, and the signal which it develops and applies to the gate of the SCR becomes too weak to turn on the SCR and the gas valve. To prevent operation of the gas valve should the thermistor become opencircuited, there is employed a second silicon-controlled rectifier circuit comprising a second SCR having anode and cathode electrodes in shunt with the thermistor and having a gate electrode which derives its voltage from across the thermistor by way of a capacitor having a predetermined minimum value sufficient to cause the second SCR to turn on before the first SCR should the thermistor become open-circuited. When the second SCR is thus turned on, the first SCR cannot be turned on, and the solenoid gas valve is thereby prevented from being turned on if the thermistor becomes open-circuited.

United States Patent [72] Inventors Douglas W. De Werth Cleveland; Earl J. Weber, Bay Village, Ohio [21] Appl. No. 782,390 [22] Filed Dec. 9, 1968 [45] Patented May 18, 1971 [73] Assignee Dover Corporation Louisville, Ky.

[54] RESISTANCE-SENSING SYSTEM WITH FAIL-SAFE CIRCUIT 7 Claims, 4 Drawing Figs.

[52] US. Cl 307/252Q, 307/310, 317/33, 317/1485, 328/3 [51] Int. Cl H03k 17/00 [50] Field of Search 307/202, 252, 310; 317/33, 148.5; 328/3 [56] References Cited UNITED STATES PATENTS 3,390,275 6/1968 Baker 307/252X Primary Examiner--Donald D, Forrer Assistant ExaminerJohn Zazworsky Attorney-Howson and Howson ABSTRACT: A control circuit suitable for use in a forceddraft gas-fired heating system permits a solenoid gas valve to be turned on only if a blower is operating. The solenoid of the gas valve is controlled by a silicon-controlled rectifier (SCR) which receives its gate signal from a thermistor circuit. The blower, when operating, blows air over the thermistor to limit the extent to which it heats up in response to current passed through it, thereby maintaining the thermistor resistance high enough to operate the SCR and the gas valve so long as the blower operates properly. If the blower does not operate properly, the thermistor warms up further, its resistance decreases markedly, and the signal which it develops and applies to the gate of the SCR becomes too weak to turn on the SCR and the gas valve. To prevent operation of the gas valve should the thermistor become open-circuited, there is em ployed a second silicon-controlled rectifier circuit comprising a second SCR having anode and cathode electrodes in shunt with the thermistor and having a gate electrode which derives its voltage from across the thermistor by way of a capacitor having a predetermined minimum value sufficient to cause the second SCR to turn on before the first SCR should the thermistor become open-circuited. When the second SCR is thus turned on, the first SCR cannot be turned on, and the solenoid gas valve is thereby prevented from being turned on if the thermistor becomes open-circuited.

s01. avg/0 M Patented May 18, 1971 3,578,987

7'0 FURNACE 4/ ,I m W867? FIG. 2A.

INVENTORS.

DOUGLAS w. DE WERTH EARL J. WEBER 9--- W WW ATTS.

RESISTANCE-SENSING SYSTEM WITH FAIL-SAFE CIRCUIT BACKGROUND OF THE INVENTION This invention relates to a resistance-sensing circuit suitable for sensing a flow of a fluid adjacent a temperature-sensitive resistive element. In its more specific aspects it relates to such a circuit for controlling the supply of gaseous fuel through a solenoid-operated valve so as to prevent turning on of the valve when an associated air-blower is not operating properly.

The above-identified type of apparatus is useful in a variety of applications. For example, it is useful in connection with a gas-fired forced-draft heating system in which it is desired to supply gaseous fuel to a gas furnace only so long as the forceddraft blower is operating. In such a system, the operation of the blower may be sensed by placing a thermistor having a negative temperature-coefficient-of-resistance in the normal path of air from the blower, and by passing an electric current through the thermistor when the blower is to be operated. If the blower does not in fact operate, the current through the thermistor will cause the temperature of the thermistor to rise substantially above room temperature, and above the normal operating temperature of the thermistor when the blower is operating properly. This rise in temperature of the thermistor will cause its electrical resistance to decrease below normal. Accordingly, the failure of the blower to operate when desired will be indicated by an abnormal decrease in the resistance of the thermistor, which may be sensed by a resistance-sensing circuit to produce a resistance-indicating signal used to prevent the opening of the valve under these conditions.

To accomplish such operation, the solenoid of the gas valve may be connected in series with the anode-cathode path of a semiconductor controlled rectifier, and the resistance-indicating signal from the thermistor may be applied between gate and cathode electrodes of the semiconductor-controlled rectifier. To produce a signal indicative of the thermistor resistance, the thermistor is connected as one leg of a voltage divider circuit across which a supply voltage is applied, to produce a heating current through the thermistor and to develop across the thermistor a voltage which is sufficiently high to turn on the semiconductor controlled rectifier only when the thermistor resistance exceeds a predetermined value which it exhibits when cooled by the blower. When the thermistor is heated above its normal temperature range due to failure of the blower to operate, its electrical resistance decreases correspondingly and the signal applied between gate and cathode of the semiconductor controlled rectifier becomes insufficient to permit operation of the solenoid gas valve.

Such a circuit in itself is subject to the limitation that if the thermistor becomes open-circuited, its apparent resistance will become and remain very high regardless of its temperature, and the resistance-indicating signal will cause the solenoid gas valve to turn on whether or not the blower is operating. Such open-circuit failure of the thermistor would therefore permit supply of gaseous fuel to the gas furnace even if the blower were not operating properly. Particularly since the thermistor is often situated at a remote location at the end of a pair of connecting leads, which may be broken inadvertently, the possibility that the thermistor may become open-circuited cannot be safely ignored. The type of circuit thus far described must therefore be considered as capable of malfunctioning in a manner such as to permit supply of gaseous fuel to the gas furnace even though the blower is not operating properly.

Accordingly it is an object of the invention to provide a new and useful resistance-sensing circuit.

Another object is to provide such a circuit incorporating certain fail-safe features.

A further object is to provide such a circuit inwhich the resistance being sensed is that of a resistor of negative temperature coefficient, and which discriminates among lower values of resistance of the resistor, medium or normal values of said resistor, and higher values of said resistor.

It is also an object to provide an electrical system for controlling actuation of a solenoid gas valve so as to prevent operation of said valve unless an air blower is operating to cool an adjacent thermistor, and in which the solenoid gas valve is prevented from opening should the thermistor become open-circuited.

A still further object is to provide a circuit of the latter type in which the solenoid gas valve is actuatable by current in the anode-cathode path of a semiconductor controlled rectifier, the gate electrode of which rectifier is supplied with actuating signals to turn on the rectifier only when the thermistor resistance increases above a predetermined level, the circuit including fail-safe circuitry so that when the thermistor becomes open-circuited the silicon-controlled rectifier is not actuated.

SUMMARY OF THE INVENTION These and other objects of the invention are achieved by the provision of a system comprising first semiconductor-controlled rectifier means, second semiconductor-controlled rectifier means, variable resistance means the resistance of which it is to be sensed, means for producing across said variable resistance means an alternating voltage which varies with the resistance value of said resistance means, a first gate circuit for connecting said variable resistance means between said first gate and cathode electrodes, a second gate circuit for connecting said variable resistance means between said second gate and cathode electrodes, means connecting said second anode and cathode electrodes in parallel with said first gate and cathode electrodes whereby occurrence of the conductive state in said second rectifier means prevents turning on of said first rectifier means, and means for sensing the occurrence of the conductive state in said first rectifier means, said second gate circuit supplying to said second gate electrode a current advanced in phase and reduced in peak amplitude compared to that supplied to said first gate electrode.

In a preferred embodiment the invention comprises first resistance means having a value of resistance variable above and below a predetermined first value, second resistance means in series with said first resistance means, means for applying an alternating voltage across the series combination of said first and second resistance means, first semiconductor controlled means having anode, cathode and gate electrodes, third resistance means connecting the gate-to-cathode path in said first semiconductor rectifier means in parallel with said first resistance means, second semiconductor controlled rectifier means having an anode, cathode and gate electrodes, means connecting the anode-to-cathode path in said second semiconductor-controlled rectifier in parallel with said first resistance means, and capacitive means connecting said gate-to-cathode path in said second semiconductor-controlled rectifier means in parallel with said first resistance means, whereby said first semiconductor-controlled rectifier is substantially nonconductive when the resistance of said first resistance means is below said predetermined first value, becomes conductive when the resistance of said first resistance means rises above said predetermined first value, and is nonconductive when the resistance of said first resistance means rises above a predetermined second value greater than said first value for which said second semiconductor-controlled rectifier becomes conductive. Where the first resistance means is a thermistor, said first semiconductor-controlled rectifier may be rendered conductive to turn on a device such as a solenoid gas valve so long as the resistance of the thermistor remains above said predetermined first value but below said second value. By placing the thermistor in the path of air from a blower, the first semiconductor-controlled rectifier can be caused to turn on to enable opening of the solenoid-controlled gas valve only when the blower is operating, and not when the blower is off or the thermistor open-circuited. Accordingly, chance open-circuiting of the thermistor leads, for example, will not permit the gas valve to be turned on by the first semiconductor-controlled rectifier means, and fail-safe operation is thereby provided.

BRIEF DESCRIPTION OF FIGURES These and other objects and features of the invention will be more readily understood from a consideration of the following detailed description, taken in connection with the accompanying drawings, in which:

FIG. 1 is a schematic diagram, partly in block form, of one preferred embodiment of the invention, and FIGS. 2A, 2B and 2C are graphical representations to which references will be made in explaining the theory and operation of the invention.

DESCRIPTION OF SPECIFIC EXAMPLES Referring now to the specific example of the invention illustrated in FIG. 1 by way of example only, there is shown a blower motor which operates an air blower I2 whenever switch 14 is closed to supply alternating current to blower motor 10 from alternating-current power source 16. Switch 14 may be a manually-operable switch, or may be a thermostatic switch for turning on the blower motor only when the ambient temperature falls below a predetermined value, as in the usual gas-fired forced-draft home heating system.

When switch 14 is closed to supply power to blower motor 10, AC voltage is also supplied, by way of stepdown transformer 20, to the transformer secondary leads 22 and 24. The latter leads are connected to opposite ends of the series combination formed by a solenoid gas valve 26 and the anode-tocathode path of a semiconductor-controlled rectifier 28. The solenoid gas valve 26 may be of any-conventional type having an electrical winding which, when supplied with sufficient electrical current, turns on the normally-closed gas valve; typically it may comprise the gas valve for supplying gaseous fuel to a gas furnace for heating air to be circulated by blower 12. In such a system it is normally important that the gas furnace not operate, and that solenoid gas valve 26 be closed, if and when blower motor 10 is not running for any appreciable length of time. Accordingly, means are provided for sensing the operation of the blower 12 and for preventing opening of the solenoid gas valve 26 unless the blower is operating properly.

The airflow sensing means utilized in this example comprises a thermistor 30 having a negative temperature coefficient of resistance, e.g. the warmer the thermistor gets the lower its electrical resistance becomes. Thermistor 30 is connected as the lower leg of a voltage divider circuit of which resistor 32 comprises the upper leg, the divider being connected between the transformer secondary leads 22 and 24. Accordingly, the voltage at tap point 34 between fixed-resistor 32 and thermistor 30 tends to vary in the same sense as the resistance of thermistor 30, and hence inversely with the temperature of thermistor 30. A gate-biasing voltage-divider circuit comprising resistors 36 and 38 in series is connected across thermistor 30, and the tap point 40 thereon is con nected to the gate electrode of the semiconductor-controlled rectifier28.

It will be understood that semiconductor-controlled rectifier 28 is of the usual type in that if its gate-tocathode voltage is zero it will remain turned off, so that there is substantially no conduction between its anode and cathode electrodes; to turn on semiconductor-controlled rectifier 28, and thereby to turn on the solenoid gas valve 26, the voltage of gate electrode 42 of controlled rectifier 28 must become positive with respect to that of its cathode electrode 44 by a substantial amount, typically of the order of one or several volts. The values of the resistors 32, 36 and 38, and the resistance of thermistor 30 are all selected in conjunction with the magnitude of the AC voltage between leads 22 and 24 to cause semiconductor-controlled rectifier 28 to be turned on during each cycle of the alternating supply voltage so long as the resistance of thermistor 30 remains above a minimum normal value R, equal to the minimum value of thermistor resistance produced when the blower is operating normally so as to cool the thermistor. However, the latter values of resistors and thermistor are preferably also selected so that the voltage produced at the gate electrode 42 of semiconductor-controlled rectifier 28 when thermistor 30 has the minimum normal resistance value R, is just sufficient to insure turning on of semiconductor-controlled rectifier 28. Accordingly, should the resistance of thermistor 30 decrease appreciably below the value R semiconductor-controlled rectifier 28 will no longer be actuated to its on state. Therefore, should the blower not operate or operate improperly when the switch 14 is closed, so that thermistor 30 is not properly cooled by the flow of air from blower 12, the resultant drop in resistance of thermistor 30 will prevent operation of semiconductor-controlled rectifier 28 and thereby also prevent opening of solenoid gas valve 26. In this way the gas furnace supplied with fuel by solenoid gas valve 26 is prevented from operating if blower motor 10 should fail to operate for any reason.

In addition to the circuit elements described in detail hereinabove, there is also employed a second semiconductorcontrolled rectifier 50 having its anode electrode 52 and its cathode electrode 54 connected in parallel circuit with thermistor 30, and having its gate electrode 56 connected by way of a series combination of resistor 58 and capacitor 60 to the tap point 34 between thermistor 30 and divider resistor 32. So long as thermistor 30 remains in the circuit as shown and has a resistance value in its normal operating range or lower, semiconductor-controlled rectifier 50, resistor 58, and capacitor 60 have no substantial effect on the previously-described operation of the circuit. However should the resistance of thermistor 30 increase above a second value R substantially higher than the normal value R,,, as will occur if thermistor 30 becomes open-circuited, semiconductor-controlled rectifier 50 will become conductive prior to the times at which rectifier 28 would normally become conductive, and in so doing will reduce the voltage at tap point 34 below that required to tire semiconductor-controlled rectifier 28; accordingly, the latter device will become nonconductive and solenoid gas valve 26 will be maintained in its closed condition. Thus, for example, if the leads to thermistor 30 are broken, semiconductor rectifier 50 will become conductive and semiconductor-controlled rectifier 28 will become nonconductive to prevent turning on of gas valve 26 and thereby discontinue operation of the gas furnace. In this way operation of the furnace is prevented unless blower motor 110 is operating and thermistor 30 is connected into the circuit properly and has a resistance value in the correct operating range.

Without thereby in any way limiting the scope of the invention, the following specific values of particular components for the circuits shown in the FIG. are given in the interest of complete definiteness. The voltage between transformer secondary leads 22 and 24 may be 24 volts r.m.s. at 60 c.p.s.; upper voltage-divider resistor 32 may have the value of about 2,200 ohms; protective resistor 36 for the gate of semiconductor-controlled rectifier 28 may have a value of about 330,000 ohms; lower voltage-divider resistor 38, which can be selected to adjust the level of signal applied to the gate of controlled rectifier 28, may have a value of about 22,000 ohms; protective resistor 58 for the gate of controlled rectifier 50 may have a value of 330,000 ohms; capacitor 60 may have a value of about 0.001 microfarad; thermistor 30 may be a Fenwal type GB4 1.11 thermistor having a resistance of about 12,000 ohms at 25 Centigrade, decreasing to about 600 ohms at about Centigrade. Solenoid gas valve 26 may have a control winding presenting a resistance of about 4.2 ohms and an impedance at 60 cycles of about 6.2 ohms. The semiconductor controlled rectifiers 28 and 50 may each comprise a General Electric type C1068] silicon-controlled rectifier. With these values, the circuit shown in the figure operates as described above when thermistor 30 is located in the airstream produced by blower l2, and serves to prevent turning on of solenoid gas valve 26 unless blower I2 is operating properly and thermistor 30 is connected into the circuit to provide resistances in the normally-to-be-expected range, as opposed to being open-circuited for example.

It has been found that the resistors 38 and 58 are not critical and in fact may be omitted. The value of capacitor 60 is relatively critical in any given application. Thus its value is limited to a predetermined range of values when used in combination with any particular set of values for the parameters of the remaining elements of the system. For the particular circuit alues given above, it has for example been found that the value oi capacitor 60 should be from about 0.0047 microfarad to about 0.0l50 microfarad. If the capacitance of capacitor 60 is substantially below this range, the controlled rectifier 50 does not fire in time to prevent turning on of rectifier 28 when the resistance of thermistor 30 is high, and hence protection against open-circuiting of thermistor 30 is not obtained; on the other hand, if the value of capacitor 60 is greater than about 0.0150 microfarad, controlled rectifier 50 remains on at all times so that gas valve 26 is never operated, thus defeating the object of supplying gaseous fuel when the blower is operating. By experimental variation the upper and lower limits of the permissible range of values of capacitor 60 may readily be determined in view of the foregoing discussion thereof.

While not wishing to be limited by the details of any particular theory, the following is believed to describe correctly certain basic principles of operation of the invention.

In the operation of the circuit, the same voltage developed at point 34 is imposed on the gate circuit of SCR 50 (capacitor 60 and resistor 58) as on the gate circuit of SCR 28 (resistor 36). This voltage is equal to the voltage drop across the thermistor 30 and may vary from zero (thermistor 30 shorted) to practically the source voltage (thermistor 30 open).

Because of capacitor 60, the gate circuit of SCR 50 has a higher impedance than that of SCR 28. Hence a higher voltage is required at point 34 to trigger SCR 50 than is required for SCR 28. As the voltage across thermistor 30 is increased (as by cooling by means of the blower), a voltage value is reached at point 34 which just triggers SCR 28. With a further increase in this voltage both SCR 28 and SCR 50 will be triggered. The fact that SCR 50, in its conducting condition, shunts out the gate circuit of SCR 28 does not stop SCR 28 from continuing to conduct provided that SCR 28 has been triggered before SCR 50. This is for the reason that an SCR, once triggered, continues to conduct for the remainder of the anode-voltage half-cycle regardless of the condition of its gate. Actually, when SCR 50 is triggered it also shunts out its own gate circuit.

In general, if SCR 28 is triggered into conduction, SCR 50 may be subsequently triggered. However, if in some other condition SCR 50 is triggered, SCR 28 cannot be subsequently triggered so long as SCR 50 remains conductive. Under the latter conditions, the shunting action of SCR 50 has reduced the gate circuit voltage of SCR 28 to zero before SCR 28 could be triggered. Such a condition exists when thermistor 30 becomes open, or when the voltage at point 34 becomes relatively high. The following explains how SCR 50 can at times be triggered before SCR 28.

The oscillating gate circuit voltage at point 34 may be described as:

50 0 sin- 28 0 =sin' V 50 28 and 0 0 =sir1 Sin This last relationship indicates that the difference between the firing angles (or firing times) becomes less as the gate circuit peak voltage, V,,, increases by virtue of increase in resistance of thermistor 30.

The gate circuit of SCR 50 contains capacitor 60 in addition to resistor 58. As a consequence, the current in this circuit leads the voltage at point 34 by an amount dependent on the relative values of elements 60 and 58. The gate circuit of SCR 28 is entirely resistive, and its current is in phase with the voltage. Therefore, the current in gate circuit SCR 50 leads the current in gate circuit SCR 28. If V is increased to a point where the firing angle difference (9 -9 becomes less than the above current lead angle, SCR 50 will trigger into conductance before SCR 28 which, in fact, cannot now be'triggered because of the shunting action of conducting SCR 50.

This same action can also be described from a consideration of current flow with the aid of FIGS. 2A, 2B and 2C, in which currents to the gate electrodes are plotted versus phase angle 9 of the currents, to a common scale. The broken-line curve is for SCR 28 while the solid curve is for SCR 50. As indicated above, the current through the gate of SCR 50 leads that through the gate of SCR 28 by some angle [3 because of the capacitor in the gate circuit of SCR 50. With any given voltage drop across thermistor 30, the amplitudes of the current curves will be inversely proportional to the impedances of the gate circuits. It is assumed that the two SCRs are quite similar and, hence, require the same firing current i,. With a very low voltage at point 34, neither SCR will receive a gate current as great as 1}. With a relatively low voltage condition as represented in FIG. 2A, only the current to SCR 28 is sufficient to exceed i and only SCR 28 fires. For the somewhat higher voltage condition shown in FIG. 2B, the firing current i, is reached at 6 for SCR 28 at which point SCR 28 becomes conducting. At some time interval later, i, is reached at 9 for SCR 50, at which time this SCR becomes conducting also, For a high voltage condition as represented in FIG. 2C, such as will occur if thermistor 30 becomes open, the gate current of SCR 50 reaches i; first at 6 at which point SCR 50 begins conducting and prevents SCR 28 from firing thereafter. Actually, the curves would collapse to zero at 9 in this figure because of the shunting action of SCR 50. With the current lead angle B fixed by the relative values of capacitor 60 and resistor 58, and with the relative amplitudes of the curves fixed by the relative impedances of the gate circuits, the crossover point of the curves is shifted to a higher current value as the amplitudes of the curves are increased by virtue of an increase in voltage, at point 34, hence the above-described result of SCR 50 firing so as to prevent SCR 28 from firing when the crossover point 100 is shifted to above i If capacitor 60 is increased in value with no other change, the current lead angle [3 is increased, as is the relative amplitude of the curve for SCR 50 because of decreased impedance in the gate circuit of SCR 50. Both of these effects shift the crossover point 100 to higher current values. Thus, capacitor 60 can be increased to a value such that the crossover point 100 will always be above i,; with this condition, SCR 28 would never fire and gas valve 26 would never open.

Conversely, and for similar reasons, a decreased value of capacitor 60 will shift the crossover point 100 downward. Thus capacitor 60 could be made so small that even the high gate circuit voltage produced with thermistor 30 open would not be sufficient to raise the crossover point 100 above i With this condition, SCR 28 will always fire to open gas valve 26 and the device will not function as intended.

It is apparent from this explanation that the same desired function of the device could be obtained by removing capacitor 60 from the gate circuit of SCR 50 and introducing an inductance into the gate circuit of SCR 28 to provide the desired relative phase difference for currents to the two gates. This modification would cause a current lag to the gate of SCR 28 relative to the current to SCR 50. Thus equivalent performance should be obtained either with a current lag at SCR 28 by virtue of an inductance in its gate circuit, or with a current lead to SCR 50 by virtue of a capacitance in its gate circuit.

Accordingly while the invention has been described with particular reference to specific embodiments thereof in the interest of complete definiteness, it will be understood that it may be embodied in a variety of forms diverse from those specifically shown and described, without departing from the invention as defined by the appended claims.

We claim:

1. Fail-safe resistance-sensing means, comprising:

first semiconductor-controlled rectifier means having first anode, cathode and gate electrodes and biased in a normally nonconductive state;

second, semiconductor-controlled rectifier means having second anode, cathode and gate electrodes, and biased in a normally nonconductive state;

means for applying an alternating voltage between the anode and cathode of each of said first and second semiconductor-controlled rectifier means;

variable resistance means, the resistance of which is to be sensed;

means for producing across said variable resistance means an alternating voltage varying with the resistance value of said resistance means;

a first gate circuit for connecting said variable resistance means between said first gate and cathode electrodes to supply said varying voltage to said first gate electrode;

a second gate circuit for connecting said variable resistance means between said gate and cathode electrodes to supply said varying voltage to said second gate electrode;

means for connecting said second anode and cathode electrodes in parallel with said first gate and cathode electrodes whereby occurrence of the conductive state in second rectifier means prevents turning on of said first rectifier means; and

means actuated by the occurrence of the conductive state in said first rectifier means;

said second gate circuit differing from said first gate circuit so as to supply to said second gate electrode a current advanced in phase and reduced in peak amplitude compared to that applied to said first gate electrode;

whereby when said varying alternating voltage is less than a first value both of said first and second semiconductorcontrolled rectifier means remain in their nonconductive states, when said varying alternating voltage is at least as great as saidfirst value but less than-a second value said first semiconductor-controlled rectifier means is rendered conductive by each cycle of said varying alternating voltage, and when said varying alternating voltage is at least as great as said second value said second semiconductor-controlled rectifier means is rendered conductive by each cycle of said varying alternating voltage to prevent said first semiconductor-controlled rectifier from becoming conductive.

2. The apparatus of claim 1, in which said alternating voltage produced across said variable resistance means and said alternating voltage applied between said anodes and cathodes of saidrectifier means are of substantially the same form and frequency.

3. Apparatus in accordance with claim 1, in which said first gate circuit comprises a first resistor in series with said first gate electrode, said second gate circuit comprises a capacitor and a second resistor in series with said second gate electrode, and the impedance of said second gate circuit of said varying alternating voltage is greater than that of said first gate circuit,

4. The apparatus of claim 1, wherein said second gate circuit comprises capacitive means for providing said advanced phase of current.

5. The apparatus of claim 1, wherein said means for producing an alternating voltage across said variable resistance means comprises a source of alternating voltage, impedance means in series with said resistance means, and means connecting said source across the series combination of said impedance means and said resistance mean s.

. Fluid flow-sensing apparatus comprising:

thermistor means having a negative temperature coefficient of resistance;

means for passing a heating current through said thermistor means to heat it and reduce its resistance, whereby a flow of a cooling fluid adjacent said thermistor means will reduce the temperature and increase the resistance of said thermistor means and increase the voltage across said thermistor means; first semiconductor-controlled rectifier means having an actuated high-conduction state, a normal low-conduction state, and first anode, cathode and gate electrodes;

first gate circuit means connecting said thermistor means to said first gate electrode to actuate said first rectifier means to said high-conduction state thereof in response to said voltage when said resistance of said thermistor means is greater than a first predetermined value but less than a second predetermined value and to permit it to assume said low-conduction state thereof when said resistance falls below said first predetermined value;

second semiconductor-controlled rectifier means having an actuated high-conduction state and a normal low-conduction state, and second anode, cathode and gate electrodes;

means for applying an alternating voltage between said first anode and cathode electrodes, and between said second anode and cathode electrode;

means connecting said first gate and cathode electrodes effectively in parallel with said second anode and cathode electrode;

second gate circuit means connecting said thermistor means to said second gate electrode and having a phase shift characteristic different than that of said first gate circuit means so as to actuate said second semiconductor-controlled rectifier means to its high-conduction state in response to said voltage across said thermistor means and to actuate said first semiconductor-controlled rectifier means to its low-conduction state when said resistance increases beyond said second predetermined value greater than said first predetermined value, whereby operation of said first semiconductor-controlled rectifier means in said high-conduction state thereof in response to opening circuiting of said thermistor means is prevented.

7. Apparatus in accordance with claim 6, in which said first gate circuit means comprises a first resistor in series with said first gate electrode, said second gate circuit means comprises a capacitor and a second resistor in series with said second gate electrode, and the impedance of said second gate circuit means to said varying alternating voltage is greater than that of said first gate circuit. 

2. The apparatus of claim 1, in which said alternating voltage produced across said variable resistance means and said alternating voltage applied between said anodes and cathodes of said rectifier means are of substantially the same form and frequency.
 3. Apparatus in accordance with claim 1, in which said first gate circuit comprises a first resistor in series with said first gate electrode, said second gate circuit comprises a capacitor and a second resistor in series with said second gate electrode, and the impedance of said second gate circuit of said varying alternating voltage is greater than that of said first gate circuit.
 4. The apparatus of claim 1, wherein said second gate circuit comprises capacitive means for providing said advanced phase of current.
 5. The apparatus of claim 1, wherein said means for producing an alternating voltage across said variable resistance means comprises a source of alternating voltage, impedance means in series with said resistance means, and means connecting said source across the series combination of said impedance means and said resistance means.
 6. Fluid flow-sensing apparatus comprising: thermistor means having a negative temperature coefficient of resistance; means for passing a heating current through said thermistor means to heat it and reduce its resistance, whereby a flow of a cooling fluid adjacent said thermistor means will reduce the temperature and increase the resistance of said thermistor means and increase the voltage across said thermistor means; first semiconductor-controlled rectifier means having an actuated high-conduction state, a normal low-conduction state, and first anode, cathode and gate electrodes; first gate circuit means connecting said thermistor means to said first gate electrode to actuate said first rectifIer means to said high-conduction state thereof in response to said voltage when said resistance of said thermistor means is greater than a first predetermined value but less than a second predetermined value and to permit it to assume said low-conduction state thereof when said resistance falls below said first predetermined value; second semiconductor-controlled rectifier means having an actuated high-conduction state and a normal low-conduction state, and second anode, cathode and gate electrodes; means for applying an alternating voltage between said first anode and cathode electrodes, and between said second anode and cathode electrode; means connecting said first gate and cathode electrodes effectively in parallel with said second anode and cathode electrode; second gate circuit means connecting said thermistor means to said second gate electrode and having a phase shift characteristic different than that of said first gate circuit means so as to actuate said second semiconductor-controlled rectifier means to its high-conduction state in response to said voltage across said thermistor means and to actuate said first semiconductor-controlled rectifier means to its low-conduction state when said resistance increases beyond said second predetermined value greater than said first predetermined value, whereby operation of said first semiconductor-controlled rectifier means in said high-conduction state thereof in response to opening circuiting of said thermistor means is prevented.
 7. Apparatus in accordance with claim 6, in which said first gate circuit means comprises a first resistor in series with said first gate electrode, said second gate circuit means comprises a capacitor and a second resistor in series with said second gate electrode, and the impedance of said second gate circuit means to said varying alternating voltage is greater than that of said first gate circuit. 